SVET Study of the Descaling of Thermal Scales Formed on AISI1095 and AISI52100 Steels

摘要

The properties of iron oxide layers formed at high temperatures on low alloy steels during industrial processes, as hot rolling and heat treatments in air or protective atmospheres, has not been much studied in the past. The same applies to the following chemical removal (descaling) of these scales, which is actually a corrosion process. For the Chemical descaling process of carbon steels hot sulfuric or hydrochloric acid solutions are industrially used. HCl-based solutions have the advantages of lower costs and higher solubility of Fe-chlorides, which is important for he following rinsing step (1).rnThe mill scale adhesion to the steel substrate before pickling is determined by its composition, thickness, and uniformity. Moreover, steel composition, and atmosphere, temperature and time in reheating furnaces, as well as, coiling rate and temperature during the hot rolling process is known to strongly affect the process of pickling (2). The formation of three iron oxides occur during the hot rolling of carbon steels in temperatures well above 570℃: Fe_(1-x)O, Fe_3O_4 and Fe_2O_3 (3,4). Fe_(1-x)O shows the fastest chemical dissolution compared to Fe_3O_4 and Fe_2O_3 (5). In low alloy carbon steels, the presence of other elements changes the composition and structure of oxide layers. We recently observed that Si and Cr form during hot rolling a barrier to oxygen diffusion, producing a more oxidized scale (6).rnThe uniform corrosion of carbon steel in NaCl solutions under open circuit conditions has the following kinetic controls: The anodic Fe dissolution is controlled by charge transfer, while the cathodic reduction of oxygen has a mixed control of mass transport and charge transfer (7). For steels covered with scales, the dissolution process during chemical descaling is for sure not a case of uniform corrosion. We may suppose that the penetration of the acid solution in scale cracks and pores will lead to the local anodic attack at this sites under cathodic reactions of hydrogen reduction at surrounding conductive scales. Scanning electrochemical techniques with high space resolution, such as Scanning Vibrating Electrode Technique (SVET) should allow the determination of the current density distribution on the corroding surface, as long as the electrolyte has a